Method and device for identity check using frequency analysis

ABSTRACT

In a method for use in checking a person&#39;s identity, at least part of an image of a fingerprint from the person is transformed so that a representation of a frequency content in said part of the image is obtained. Based on the obtained representation of the frequency content, a frequency code is determined, which contains a predetermined number of frequency code values. A device for carrying out the method is also described.

FIELD OF THE INVENTION

The present invention relates to a method and a device for use inchecking a person's identity.

BACKGROUND ART

It is known to use fingerprints for checking a person's identity. Insuch a check, current data from a current fingerprint from the personwhose identity is to be checked is compared with previously recordedreference fingerprint data (below sometimes only called reference data)for one or more persons.

If the check concerns a verification of the person's identity, thecurrent data from the current fingerprint is compared only withreference data for the person who the person whose identity is to bechecked pretends to be.

If the check concerns an identification of the person's identity, thecurrent data is compared with reference data for at least two, butusually several, different persons to determine whether the currentfingerprint originates from any of these persons.

Reference fingerprint data for a plurality of persons can be stored in alocal database for an individual fingerprint checking system or in acentral database for a plurality of checking systems. Alternatively,reference data for a person can be stored in a personal unit, such as apersonal memory card or smart card, which the person uses when he or shewants to authenticate himself/herself, i.e. prove his/her identity byverification or identification.

In some types of storing units there may be a limited storage capacity.In such cases it is desirable to have a small amount of reference data.

The comparison between reference data and current data can be madeeither in the same physical unit as the one in which the reference datais stored, in the same physical unit as records and processes thecurrent fingerprint or in some other unit.

If the comparison is made in a unit with a limited processor capacity,it is desirable that the comparison between current data and referencedata can be made in a simple way. If either current data or referencedata has to be transferred from one unit to another in connection withthe comparison, it is desirable for the amount of data to be small sothat the transfer time will be short.

In many applications, the user wants to be able to authenticate himselfand get access to a protected object without delay. Then it is desirablethat the current data can be generated quickly, that the transfer time,if any, is short and that the comparison can be made quickly.

The reference data can correspond to a complete fingerprint as recorded.However, only part of the information in the fingerprint is usuallysaved as reference data.

For instance, it is known to save as reference data information aboutspecific features, also referred to as minutiae points, in thefingerprint. These specific features are usually of two predeterminedtypes, viz. fingerprint ridge endings and fingerprint ridgebifurcations. For instance, coordinates for where these features areplaced can be saved as reference data. When checking a person'sidentity, the relative location of features in a current fingerprint isdetermined and then compared with the relative location of the featuresin the reference data.

From, for instance, WO 01/84494 it is also known to save as referencedata partial areas of an image of a reference fingerprint. When checkinga person's identity, corresponding partial areas are found in a currentfingerprint and then compared with the partial areas in the referencedata. To allow the final identity check to be carried out on a smartcard with a limited processor and memory capacity, a partial area isstored as a so-called public partial area in a public part of thereference data on the smart card. In the identity check, this publicpartial area is read to a computer unit, in which it is compared with acurrent fingerprint to determine in which position in relation to thisthe public partial area fits best. In this way, the reference data isaligned with the current fingerprint. In the public part of thereference data, also coordinates for how other partial areas, so-calledprivate partial areas, are placed in relation to the public partial areaare stored. When the current fingerprint has been aligned with thereference data, these coordinates can be used to determine which partialareas in the current image are to be selected and sent to the smart cardfor comparison with the private partial areas.

It is further known to form a finger code by first finding an objectivereference point in a fingerprint, then dividing the fingerprint intosectors in relation to the reference point, then Gabor filtering theimage with a number of Gabor filters and finally calculating the fingercode as the variance in each sector of the Gabor-filtered image. Theobtained finger code is advantageous as reference data since it will bequick and easy to compare a reference finger code with a current fingercode. However, it takes relatively long to find the finger code. Anotherproblem is that it is based on the fact that an objective referencepoint can be established in the fingerprint merely by searching in thesame. It is known, however, that for certain fingerprints it isdifficult, not to say impossible, to establish such an objectivereference point.

SUMMARY OF THE INVENTION

An object of the invention is to provide a method and a device for usein checking a person's identity, which method and which device make itpossible to wholly or partly satisfy one or more of the above desiderataas regards storage space, transfer time, comparison time and time forgenerating data.

More specifically, according to the invention, a method is provided foruse in checking a person's identity, comprising the steps oftransforming at least part of an image of a fingerprint from the personso that a representation of a frequency content in said part of theimage is obtained, and based on the obtained representation of thefrequency content, determining a frequency code which contains apredetermined number of frequency code values.

There are techniques, for instance Fourier transforming, which make itpossible to quickly transform an image so that a representation of thefrequency content in the image is obtained. Based on thisrepresentation, a frequency code can be determined relatively quicklyand easily. It is also possible to generate reference data and currentdata in a time which is acceptably short for many applications.

Depending on how many frequency code values the frequency code containsand how the frequency code values are defined, it is possible to producereference data and current data requiring a small storage space andshort transfer time, but still giving a probability of error which isacceptably low for many applications regarding errors of type 1 (that acurrent fingerprint from the same person and finger as those from whichthe reference data is fetched is not accepted) and of type 2 (that acurrent fingerprint from another person or another finger than the onefrom which the reference data is fetched is accepted).

As mentioned above, the method according to the invention is intendedfor use when checking a person's identity. This may comprise, forinstance, that the method is used for producing reference fingerprintdata for a person, for producing current fingerprint data or forchecking a person's identity.

The image can be a grey-scale image, a binary image or a colour image.It shows a fingerprint by representing ridges and valleys in thefingerprint with different intensities. The image may comprise acomplete fingerprint or a partial fingerprint depending on the size ofthe sensor that has been used to record the image.

By the image or part of this being transformed is here meant that amathematically defined transform is allowed to operate on the image andconvert the information in the image into a representation of thefrequency content, i.e. with which frequencies it is possible todescribe the intensity variations in the image in different directionsand the mutual relationship thereof.

In some cases, it may be desirable to transform only part of thefingerprint image. It may be desirable, for instance, to exclude thebackground or parts where the fingerprint is of poor quality, it may bedesirable to use only an area round a certain point, or it may for someother reason be desirable to select only a part to be transformed.

The frequency code contains a predetermined number of frequency codevalues. The number of values in the code can be determined from thedesired size of the reference data and the desired probability of errorof types 1 and 2. A greater number of values requires a greater storagecapacity for the reference data, but normally gives lower probability oferror.

The transform can be, for instance, one, or a combination, of thefollowing transforms: Fourier transforms, Cosinus transforms, Besseltransforms or Hadamard transforms. An advantage of these transforms isthat they are to some extent insensitive to lateral displacements of afinger on a sensor as long as the same part of the finger is located onthe sensor.

The frequency code values are determined based on the obtainedrepresentation of the frequency content. More specifically, they can bedetermined based on a predetermined number of frequency values that areselected from the representation of the frequency content, in which caseeach frequency value may be a measure of the existence of acorresponding frequency in a certain direction in the image that istransformed. One or more frequency values can be used to produce afrequency code value. A plurality of frequency values can, for instance,be averaged or weighted together to determine a frequency code value.

The method according to the invention can be accomplished by means of acomputer program. The computer program can be stored on a storagemedium, for instance in a memory of electronic, optical, magnetic orsome other known type. The storage medium can also be a propagatingsignal.

According to another aspect of the invention, a device is provided foruse in checking a person's identity, comprising a signal processor,which is adapted to transform at least part of an image of a fingerprintfrom the person so that a representation of a frequency content in saidpart of the image is obtained, and based on the obtained representationof the frequency content, determining a frequency code which contains apredetermined number of frequency code values.

The signal processor can be accomplished by means of a suitablyprogrammed general or specially adapted computer. It can alternativelybe accomplished with specially adapted hardware, such as an ASIC(Application Specific Integrated Circuit) or with an FPGA (FieldProgrammable Gate Array) or with analog circuits or digital circuits orwith a suitable combination thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in more detail by way ofexample and with reference to the accompanying drawings, in which

FIG. 1 shows most schematically the basic principle of the presentinvention;

FIG. 2 shows an example of a system in which an identity check by meansof frequency codes can be performed;

FIG. 3 shows an image of a fingerprint;

FIG. 4 shows a representation of the frequency content in thefingerprint in FIG. 3;

FIG. 5 is a flow chart and shows an example of a method for producingreference fingerprint data; and

FIG. 6 is a flow chart and shows an example of a method for identitychecking.

DETAILED DESCRIPTION OF EMBODIMENTS

The present invention is based on the idea of generating a frequencycode that can be used when checking a person's identity. This isschematically illustrated in FIG. 1.

First, an image 1 of a fingerprint from a person is recorded. This imagerepresents the information in the fingerprint in the form of intensityvariations. The image 1 is transformed 2 so that a representation 3 ofthe frequency content in the image 1 is obtained instead.

From this frequency representation, a number of frequency values isselected and processed 4 so that they can be represented in a compactway in the form of frequency code values. The frequency code valuesjointly form a frequency code C_(R). This frequency code can be storedas reference fingerprint data to be used in a subsequent identity check.

In the identity check, a current frequency code C_(A) is generated froma current fingerprint and compared with the previously stored referencefrequency code C_(R). If the compared frequency codes are sufficientlyequal, the fingerprints from which they are generated will be consideredto originate from one and the same finger, and thus the person'sidentity is ensured.

In the following, an example of a system will be described, in which anidentity check by means of frequency codes can be performed. The systemcomprises, as shown in FIG. 2, a fingerprint sensor 10 for recordingfingerprints, a first unit 11, which in the following is called computerunit, for processing fingerprint data, and a second unit 12, whichcomprises a memory for storing reference fingerprint data and aprocessor for processing fingerprint data and which in this caseconsists of a smart card.

The sensor 10 can, but need not, be used both for recording of referencefingerprints and for recording of current fingerprints. It can beoptical, capacitive, thermal or be of some other convenient type. It canbe an area sensor or a line sensor.

The computer unit 11 can be a common general computer, such as a PC.Alternatively, it can be, for instance, a computer unit 11 which isspecially adapted for this application or specially adapted hardware. Inthis example, it comprises a smart card reader 13, which may be anycommercially available smart card reader or a specially designed/adaptedsmart card reader. The smart card reader 13 may be physically integratedinto the computer unit 11 or may be arranged in a casing of its ownwhich in terms of signals is connected or connectible to the rest of thecomputer unit. There may be one or more processors in the computer unit11, and the processing of fingerprint data that takes place in thecomputer unit can be distributed in different ways among differentprocessors.

The smart card 12 can be any type of smart card on which a comparison offingerprint data is to be carried out. The smart card 12 has a signalprocessing unit which comprises a processor 16, a memory 17 for storingof reference fingerprint data which is extracted from a referencefingerprint from the smart card holder, and a working memory 18, as wellas communication circuits 19 which enable communication between thesmart card reader 13 and the smart card 12. The communication circuits19 can, but need not, require contact between the smart card 12 and thereader 13.

In the following, an example of how the invention can be realised in thesystem shown in FIG. 2 will be described.

To allow the smart card 12 to be used to verify the smart card holder'sidentity, reference fingerprint data must be stored in the memory 17 ofthe smart card. This is preferably carried out under such conditionsthat it is possible to ensure that it is really the smart card holder'sreference fingerprint data that is stored. An example of how therecording of reference fingerprint data is made is shown in the flowchart in FIG. 5.

In a first step 50, an image of the smart card holder's fingerprint isrecorded by means of a sensor 10. An example of how a real fingerprintimage that has been recorded by means of a capacitive fingerprint sensoris shown in FIG. 3. In the image, the dark lines correspond to ridges inthe fingerprint and the light lines to valleys in the fingerprint. Thedarker portions that are found in the upper and lower edge of the imageare caused by remaining layers of fat on the sensor.

The image is read from the sensor 10 into the computer unit 11 where itis Fourier transformed, step 51. The Fourier transformation can be madeby means of specially developed software or hardware or by means ofcommercially available software or hardware. It is normally made bymeans of calculations based on a mathematical method, but can also bemade without calculations using an optical lens and a sensor.Optionally, only part of the image can be selected and transformed.

By the Fourier transformation, frequency values F(u,v) are obtained,which consist of complex numbers with a real part and an imaginary partas follows

${F\left( {u,v} \right)} = {\frac{1}{M\; N}{\sum\limits_{x = 0}^{M - 1}\;{\sum\limits_{y = 0}^{N - 1}\;{{f\left( {x,y} \right)}{\exp\left\lbrack {{- {j2}}\;{\pi\left( {\frac{u\; x}{M} + \frac{v\; y}{N}} \right)}} \right\rbrack}}}}}$

-   -   where u=0,1,2, . . . , M-1        -   v=0,1,2, . . . , N-1            and where u and v are Cartesian coordinates in the Fourier            transformed image and x and y are Cartesian coordinates in            the fingerprint image (FIG. 3).

The frequency values F(u,v) constitute a representation of the frequencycontent in the fingerprint image.

The frequency content can be illustrated in an image.

FIG. 4 shows a Fourier transform of the image in FIG. 3. Morespecifically, the absolute values of the frequency values F(u,v) areshown. FIG. 4 shows a grey-scale image of what frequencies exist indifferent directions in the fingerprint in FIG. 3, the frequencyincreasing as a function of the radius r in FIG. 4, the angle θ from thehorizontal axis A in FIG. 4 indicating the direction in FIG. 3 andhigher existence of a certain frequency being illustrated in lightergrey-scale in FIG. 4. The representation in FIG. 4 is symmetrical sincethe frequency content will be the same in directions that differ by 180degrees.

For each frequency in FIG. 4 there is thus a frequency value which givesa measure of the existence of the frequency in the current direction v.These frequency values are used to generate the frequency code. Morespecifically, a predetermined number of frequency values is selected,step 52.

FIG. 4 shows all frequencies within a certain frequency range, i.e. alsofrequencies that are not significant to a fingerprint since they do notat all reflect the closeness of fingerprint lines in differentdirections in a fingerprint. Therefore the frequency values are selectedfor frequencies within a limited range in FIG. 4, which range isrelevant for fingerprints. In FIG. 4, frequencies are selected in theupper semi-plane in the annular area between the radii R₁ and R₂.

The relevant range is individual-dependent since different people havedifferent spacings between the fingerprint lines. The frequency valuescan then be selected either for frequencies within a large predeterminedrange taking the distribution among different individuals intoconsideration, or for frequencies within a smaller individual-adaptedrange. The individual-adapted frequency range is determined as a rangein which the individual in question has a high existence of relevantfrequencies, i.e. frequencies corresponding to structures in thefingerprint.

For instance, the frequency range can in the former case be selected soas to correspond to 800 to 5000 lines/m.

For determining the frequency code values, either the real parts, theimaginary parts, the absolute values or a combination thereof can beused, for instance phase displacement.

In this example, it is assumed that the absolute values (also calledmagnitudes) of the selected frequency values are calculated, step 53.

The frequency code C is generated by the sequence of the thus generatedfrequency code values. Expressed in a different way:C(r,v)=|F(r,θ)| where R ₁ <r<R ₂ and 0<v<πwhere F(r,θ) is the frequency value in the point (r,θ) and C(r,θ) is thefrequency code value for the same point.

With this terminology, it is thus the frequency code values C(r,θ) thatare shown in FIG. 4. Thus it is possible, as an alternative, first tocalculate the frequency code values for all points and then selectfrequency code values for predetermined frequencies as the frequencycode values to form to the frequency code.

For the frequency code to be represented in a memory-saving manner, thefrequency code values C(r,θ) are then quantized, step 54, for instanceto one of the values 0, 1, 2 or 3.

If 800 frequency values are selected, for instance 20 differentfrequencies for each of the 40 different angles v, a frequency codeC_(R) with 200 byte data will be obtained if the frequency code valuesare quantized as described above.

The values are stored in a table with 5 bytes in each row. This meansthat each row contains the quantized frequency code values for anangular value. A change of the frequency code corresponding to a turningof the fingerprint in either direction at an angle of 4.5 degrees canthus easily be achieved. The only thing that need be made is tocircular-shift the rows in the table one step in the correspondingdirection.

Finally, the frequency code C_(R) is stored, step 55, as referencefingerprint data on the smart card 12. As will be described in moredetail below, the frequency code can be stored as public referencefingerprint data that is allowed to be read from the smart card for usein the computer unit in an identity check, or as private referencefingerprint data that is not allowed to be read from the smart card butis compared with current fingerprint data on the smart card.

Once reference fingerprint data has been stored on the smart card 12,the smart card holder can use the smart card to authenticate himself. Anexample of how this may be done will be described in the following.

The smart card holder inserts his smart card 12 in the smart card reader13 and places the same finger on the sensor 1 as was used for recordingof reference fingerprint data on the sensor 10.

The sensor 10 records a current image of the smart card holder'sfingerprint, step 60 in FIG. 6. The image is read into the computer unit11 where it is processed in exactly the same way as in the recording ofthe reference fingerprint data. More specifically, the image is Fouriertransformed, step 61, frequency values are selected in the image, step62, the absolute values of the selected frequency values are calculated,step 63, and the calculated absolute values are quantized, step 64, toform the current frequency code C_(A).

As an alternative, the frequency values can be selected within apredetermined, individual-independent range. This alternative may beused even if the reference frequency code is based on anindividual-dependent range since in that case a greater currentfrequency code can be sent to the smart card and a suitable subset ofthe current frequency code can be selected on the smart card based ondata stored there regarding on what frequency range the referencefrequency code is based.

As another alternative, the frequency values can be selected individualdependent. The frequency range may then be selected depending on wherein the representation of the frequency content in the currentfingerprint the individual has high existences of frequencies.Alternatively, the frequency range may be selected based on data aboutthis that is sent from the smart card.

The current frequency code C_(A) is sent to the smart card 12 where itis compared, step 65, with the previously stored reference frequencycode C_(R). The comparison is made by determining the absolutedifference between frequency code values corresponding to each other.The total difference is then compared, step 66, with a threshold valueto determine whether a similarity condition is satisfied and whether thereference frequency code C_(R) and the current frequency code C_(A) canthus be considered to originate from one and the same finger.

In the recording of the current fingerprint it may happen that theperson in question places his finger at a different angle on the sensorcompared with the recording of the reference fingerprint. This willaffect the frequency code by the representation of the frequency contentbeing rotated. The frequency code values will thus be in a differentorder in the frequency code. In the identity check, this can be takeninto consideration by repeating the comparison between the currentfrequency code and the reference frequency code with a mutualpermutation of the frequency code values, step 67. The mutualpermutation can be effected in various ways; the processor on the smartcard can permute the values either in the reference frequency code or inthe current frequency code so that the permutation corresponds to acertain degree of rotation or, alternatively, the reference frequencycode can be stored on the smart card in different versions with thefrequency code values permuted. As another alternative, the computerunit 11 can generate different versions of the current frequency codeand send them to the smart card for comparison.

In the case when several comparisons are made, the criterion foridentifying a person will be that one of the permutations satisfies thesimilarity criterion.

However, the permutation need not always be effected. Some fingerprintsensors have different control means which command the user to alwaysplace his finger in the same position on the sensor. When using such afingerprint sensor, the frequency code thus need not be permuted.

It may also happen that the user when recording the current fingerprintplaces his finger in the same rotational position on the sensor but in atranslated position. Since the frequency code is based on the frequencycontent, which is a global property of the fingerprint, the translationwill have a smaller effect on the frequency code compared with the caseof using minutiae points and partial areas representing local propertiesin the fingerprint.

An embodiment has been described above where the current frequency codeis generated in the computer unit and then compared with a referencefrequency code on the smart card. In this case, the reference frequencycode never leaves the smart card and therefore constitutes privatereference fingerprint data.

However, it is also conceivable to use the reference frequency code aspublic reference fingerprint data which is allowed to be read from thesmart card for use in the computer unit for alignment of the currentfingerprint image with the private reference fingerprint data so thatthe final comparison between current fingerprint data and referencefingerprint data will be easier and quicker to perform.

More specifically, in this case the reference fingerprint code is thusstored as public reference data. If the reference fingerprint code isbased on the absolute values of the frequency values, alignment can onlybe made in the rotational direction. On the other hand, if the referencefingerprint code contains both quantized real parts and quantizedimaginary parts of the frequency values, alignment may however also bemade in the translational direction.

Moreover, other fingerprint data is stored as private reference data onthe smart card. This other fingerprint data may consist of a fingerprintcode which is based on other points in the representation of thefrequency content, partial areas of the reference fingerprint image,minutiae points from the reference fingerprint image or some otherfingerprint data which is determined from the reference fingerprintimage.

In the identity check, a current image of the fingerprint from theperson whose identity is to be checked is recorded in the same way asbefore. The current image is Fourier transformed as described above. Thecomputer unit further reads the public reference fingerprint data fromthe smart card, i.e. in this case the reference frequency codeC_(R)(r,θ). Each complex value of the Fourier transform is thenmultiplied by the corresponding complex value in the reference frequencycode C_(R)(r,θ) as followsG(r,θ)=F(r,θ)*C _(R)(r,θ).

According to the correlation theorem, this multiplication corresponds toa correlation of two images in the spatial plane.

After that G(r,θ) is inverse Fourier transformed back to the spatialplane to g. The coordinates for the maximum value of g then immediatelycorresponds to the relative translation between the referencefingerprint image and the current fingerprint image. If these are notdisplaced in relation to each other, the maximum value of g would thusbe in the centre of g.

The rotation between the current image and the reference image canfurther be determined by permutation of the reference frequency code anda current frequency code which is generated from the Fourier transformin the same way as described above. Once the rotation and thetranslation have been determined, the current fingerprint image and thereference fingerprint image have thus been aligned with each other.Starting from the alignment, data from the current image can be moreeasily compared with reference data on the smart card.

If the private reference data consists of partial areas, the currentpartial areas can be determined after the alignment has been made bymeans of the frequency code. If the private reference data consists ofminutiae points, coordinates for these can be determined in thecoordinate system of the reference data after alignment by means of thefrequency code.

As a further alternative, comparison by means of the frequency code canbe carried out supplementing a comparison between some other type offingerprint data. Since the frequency code reflects a global property ofthe fingerprint, it may be convenient to make a supplementary comparisonof a local property, for instance minutiae points or partial areas. Thisshould increase safety in the authentication. The criterion for thecurrent person's identity to be considered authenticated can be that anauthentication threshold value should be achieved separately for bothcomparisons of the fingerprint data or be achieved for only onecomparison. Alternatively, it is possible to use some kind of criterionwhich is based on a weighting of the authentication threshold values forboth comparisons.

An embodiment has been described above where the frequency code isgenerated in a computer unit and where the comparison takes place on asmart card. It is possible, however, to realise the invention by meansof some other pair of a first and a second unit. The first unit may be,for instance, a PC of an Internet customer and the second unit acomputer of a service provider which stores the reference fingerprintdata under safe conditions. As a further example, the first unit mayconsist of a mobile phone and the second unit of a SIM card or the likein the mobile phone. It is also conceivable that the method can berealised wholly or partly in some other type of portable unit, such as aPDA, a pen provided with a processor, or an access protected hard disk.

It is alternatively conceivable to carry out the method described aboveby determining a current frequency code and comparing the currentfrequency code with a reference code in one and the same unit.

A further conceivable application is to use the frequency code inidentification. The current frequency code is then compared withreference frequency codes for a number of people. The purpose is then toselect in a first step a small number of the people as candidates foradditional comparison. The additional comparison can then be made bymeans of some other type of reference fingerprint data, for exampleminutiae points or partial areas.

1. A method for use in checking a person's identity in a unit having aprocessor, comprising the steps of: transforming by said processor atleast part of an image of a fingerprint from the person so that arepresentation of a frequency content in said part of the image isobtained; selecting by said processor, from the representation of thefrequency content, a predetermined number of frequency values, whereinsaid selecting is made for frequencies within a smallerindividual-adapted range such that said frequencies correspond tospacing between fingerprint lines of the person; determining by saidprocessor frequency code values based on the frequency values, whereinthe determination of said frequency code values comprises quantizing theselected frequency values; and determining by said processor a frequencycode which contains a predetermined number of frequency code values. 2.A method as claimed in claim 1, wherein the step of transforming isperformed by means of at least one transform from the group of Fouriertransforms, Cosinus transforms, Bessel transforms and Hadamardtransforms.
 3. A method as claimed in claim 1, wherein the frequencyvalues selected are frequency values of predetermined frequencies.
 4. Amethod as claimed in claim 1, comprising selecting by said processor, inthe representation of the frequency content, an individual frequencyrange for the person, said predetermined number of frequency valuesbeing selected for frequencies in the selected frequency range.
 5. Amethod as claimed in claim 1, comprising calculating by said processorthe absolute value of at least one of the selected frequency values andusing the absolute value for determining one of said frequency codevalues.
 6. A method as claimed in claim 1, comprising using by saidprocessor, for at least one of the selected frequency values, the realpart of the selected frequency value for determining one of saidfrequency code values.
 7. A method as claimed in claim 1, comprisingusing by said processor, for at least one of the selected frequencyvalues, the imaginary part of the selected frequency value fordetermining one of said frequency code values.
 8. A method as claimed inclaim 1, wherein the frequency content is represented in polarcoordinates.
 9. A method as claimed in claim 1, comprising storing thefrequency code as reference fingerprint data for the person.
 10. Amethod as claimed in claim 9, wherein the reference fingerprint data isstored on a portable data carrier.
 11. A method as claimed in claim 9,wherein the reference fingerprint data is stored in a unit as publicreference fingerprint data that is allowed to be read from the unit. 12.A method as claimed in claim 9, wherein the reference fingerprint datais stored in a unit as private reference fingerprint data that is notallowed to be read from the unit.
 13. A method as claimed in claim 1,wherein the frequency code is compared with at least one referencefrequency code which contains a predetermined number of frequency codevalues.
 14. A method as claimed in claim 12, comprising repeating bysaid processor the comparison between the frequency code and thereference frequency code with a mutual permutation of the frequency codevalues.
 15. A method as claimed in claim 12, further comprising the stepof permuting by said processor the frequency code values in either thefrequency code or the reference frequency code and repeating thecomparison.
 16. A method as claimed in claim 12, wherein the frequencycode is determined in a first unit and the comparison with the referencefrequency code takes place in a second unit.
 17. A method as claimed inclaim 12, wherein the frequency code is determined in a first unit, thereference frequency code is received from a second unit, the frequencycode is compared with the reference frequency code and other data fromthe image is sent to the second unit based on the result of thecomparison between the frequency code and the reference frequency code.18. A method as claimed in claim 12, wherein the frequency code iscompared with a plurality of reference frequency codes for differentpeople, and a small number of said different people is selected forfurther checking.
 19. A computer-readable storage medium storing programinstructions that, when executed by a processor, perform a method foruse in checking a person's identity, the method comprising the steps of:transforming at least part of an image of a fingerprint from the personso that a representation of a frequency content in said part of theimage is obtained; selecting, from the representation of the frequencycontent, a predetermined number of frequency values, wherein saidselecting is made for frequencies within a smaller individual-adaptedrange such that said frequencies correspond to spacing betweenfingerprint lines of the person; determining frequency code values basedon the frequency values, wherein the determination of said frequencycode values comprises quantizing the selected frequency values; anddetermining a frequency code which contains a predetermined number offrequency code values.
 20. A device for use in checking a person'sidentity, comprising a signal processor which is adapted to: transformat least part of an image of a fingerprint from the person so that arepresentation of a frequency content in said part of the image isobtained; select, from the representation of the frequency content, apredetermined number of frequency values, wherein said selection is madefor frequencies within a smaller individual-adapted range such that saidfrequencies correspond to spacing between fingerprint lines of theperson; the signal processor is further adapted to determine frequencycode values based on the frequency values, wherein the determination ofsaid frequency code values comprises quantizing the selected frequencyvalues; and determine a frequency code which contains a predeterminednumber of frequency code values.
 21. A method as claimed in claim 2,comprising selecting, from the representation of the frequency content,a predetermined number of frequency values, and determining saidfrequency code values based on the frequency values.
 22. A method asclaimed in claim 9, wherein the reference fingerprint data is stored ona smart card.
 23. A method as claimed in claim 10, wherein the referencefingerprint data is stored in a unit as public reference fingerprintdata that is allowed to be read from the unit.
 24. A method as claimedin claim 10, wherein the reference fingerprint data is stored in a unitas private reference fingerprint data that is not allowed to be readfrom the unit.
 25. A method for use in checking an identity of a personin a unit having a processor, comprising the steps of: transforming bysaid processor at least a part of an image of a fingerprint from theperson so that a representation of a frequency content in said part ofthe image is obtained; selecting by said processor, from therepresentation of the frequency content, a predetermined number offrequency values, wherein said selecting is made for frequencies withinan individual-adapted range such that said frequencies correspond tospacing between fingerprint lines of the person; determining by saidprocessor frequency code values based on the frequency values; anddetermining by said processor a frequency code which contains apredetermined number of frequency values.